Recently, with the widespread use of portable electronic devices, electric vehicles, and so on, developments of power storage devices, including lithium-ion secondary batteries and sodium-ion secondary batteries, have been promoted. Materials containing Si or Sn having a high theoretical capacity are being studied as negative-electrode active materials for use in power storage devices. However, with the use of a negative-electrode active material containing Si or Sn, the negative-electrode active material exhibits a large volume change due to expansion and contraction thereof occurring during insertion/extraction reaction of lithium ions or sodium ions, which presents a problem that there arises a significant collapse of the negative-electrode active material due to repeated charges and discharges and, therefore, the cycle characteristics are likely to decrease.
To cope with this, NaTi2(PO4)3 and Na3Ti2(PO4)3, which are NASICON-type compounds, are proposed as negative-electrode active materials having relatively good cycle characteristics (see, for example, Patent Literature 1 and Non-Patent Literature 1).